An automobile of FR drive model or four-wheel drive model includes a power transmission, which transmits the rotational driving force of the engine to the drive wheels, and it generally has an arrangement in which the rotational driving force is transmitted from the transmission through a propeller shaft to an axle device. For example, the engine is placed in the front part of the vehicle, and the output rotation of the transmission, which is coupled to the engine, is transmitted through the propeller shaft to the rear axle device. Various such power transmissions have been known, and some are disclosed in Japanese Laid-Open Patent Publication No. H10(1998)-194004 and in Japanese Laid-Open Utility-Model Publication No. H07(1995)-8632.
By the way, if a rotational member of the power transmission is unbalanced, then there is a problem of vibration that can occur because of the unbalance while the rotation is being transmitted. Especially, the propeller shaft, which is a long cylindrical member that extends longitudinally in the vehicle, is likely to experience a rotational unbalance. Another element that can cause a rotational unbalance is a coupling device, which is used for connecting the propeller shaft to the axle device. The coupling device comprises as components a yoke flange, a companion flange and the like, which are forged parts and not in perfect rotational configuration (not in ring figure or in cylindrical figure).
To solve this problem, conventionally, various methods are adopted to prevent occurrence of rotational unbalance (for example, refer to Japanese Laid-Open Patent Publication No. H07(1995)-167219). Particularly, an unbalance-offsetting method has been practiced in recent years. In this method, the unbalances are determined individually for the propeller shaft and for the companion flange of the axle device before their assembly in the vehicle. A point where one of the unbalanced rotating masses, i.e., either the propeller shaft or the companion flange, becomes heaviest (heavy point) is defined and marked, and a point where the other of the unbalanced rotating masses becomes lightest (light point) is defined and marked. When the propeller shaft and the companion flange are connected to each other in the assembly process of the vehicle, the heavy point and the light point are phased to offset the rotational unbalances.
In such a method, which adjusts the unbalances of the propeller shaft and the companion flange during the assembly process, while a worker is confirming that the two markings of the heavy and light points are within a predetermined phase angle, he must position the holes for the insertion of stud bolts for the connection of the propeller shaft and the companion flange. Therefore, the workability of this method is terribly bad. Moreover, it is difficult to determine whether or not the two markings of the heavy and light points are within a predetermined angle, so there has been a problem that relatively small deviations of these markings are not determinable. In a conventional technique that attempts to solve this problem, the holes for the insertion of bolts are positioned to make the two markings of the heavy and light points closest to each other before the fastening of the bolts. However, the propeller shaft and the companion flange have each, for example, n bolt holes, so there is a possibility of phase deviation for the markings within a range of +360/n to −360/n degrees in the assembly process. This range for the phase deviation has been so great to jeopardize the effectiveness of the above mentioned unbalance-offsetting method (refer to Japanese Laid-Open Patent Publication No. 2002-104002).